1. Field of the Invention
This invention relates to slotted waveguide antennas, and is more particularly directed to microwave antennas formed as an array of extruded waveguide elements.
2. Description of the Prior Art
Waveguide antennas are well known, and many have been proposed in which an array of waveguide elements each have slots in one face to serve as radiating or receiving elements. These antennas are configured in planar arrays, with up to one hundred lengths of waveguide stacked one on top of the other, and each of the waveguide elements having one hundred or more slots along its length. The waveguide elements are typically of rectangular cross section and are stacked together and mounted on a backing plate. The slots are cut on a face of the waveguide remote from the backing plate. The common practice is to form the waveguide elements with stiffener or reinforcing flanges along the broad walls. These can connect to one another and overlap, or can be attached to an additional support element on the backing plate. These flanges are typically positioned one quarter wavelength back from the slotted face, considering the band of wavelengths for which the antenna is designed. This creates a short circuit position between successive superposed waveguide elements at the quarter-wavelength distance.
Examples of prior art waveguide antennas are shown in U.S. Pat. No. 4,255,752 to Noble et al. and U.S. Pat. No. 4,517,571 to Mulliner. In addition, a lightweight antenna has been proposed which employs specially extruded waveguides with flanges both at the rear of each waveguide element and also at the quarter wavelength short-circuit position. The flanges of the rear of the waveguide are overlapped and riveted together. This produces a virtually self-supporting antenna face. This antenna is suitable for use with microwave frequencies below X-band and up to X-band, provided full height waveguide is employed. However, for reduced height waveguides and for frequencies at X-band and above, there is insufficient space between elements to accommodate rivets or other mechanical fasteners.
A self supporting multiband array antenna is proposed in Nemit et al., U.S. Pat. No. 4,243,990. Wherein a plurality of waveguide elements are spaced apart from a conductive backplate by spacers. However accurate prealignment of the elements with respect to one another and the backplate during fabrication of the array is not possible with this arrangement.